The present invention relates to an improved electrical discharge machining apparatus for use particularly in precision machining in which displacement between the workpiece and the electrodes due to changes in machining power or environmental conditions is prevented.
A conventional apparatus of the same general type to which the invention pertains is shown in FIG. 1. In this figure, reference numeral 1 indicates a machining tank, 2 a machining liquid, 3 an electrode, 4 a spindle for moving the electrode 3 in the vertical direction, 5 a workpiece, and 6 a table supporting the machining tank 1 and the workpiece 5, the table being movable in the X direction as indicated by an arrow in the figure. In the same figure, 7 represents a saddle on which the table 6 can be moved in the Y direction shown by an arrow in the figure, the Y direction being at a right angle to the X direction and together with the X direction defining the horizontal plane. Reference numeral 8 indicates a head within which the spindle 4 slides and is protected, 9 a column supporting the head 8, 9a the front side of the column, 9b the back side of the column, 10 a bed on which the column 9 is fixed and supported, and 12 an air blower fixed at the top of the column 9.
The following is a description of the operations of this unit. The workpiece 5 is machined by an electrical discharge generated between the electrode 3 and the workpiece 5 within the machining liquid 2 in the machining tank 1 when a voltage is applied from a power source (not shown in the drawing). Because of the discharge energy, the temperature of the machining liquid 2 rises, whereby the temperature of the machining tank 1, which is heated by the liquid, and that of the table 6, rise. The temperature increases gradually at a rate dependent on the machining power. Heat 11, which is radiated as shown by arrows in the figure, is generated from the surface of the machining liquid 2, the machining tank 1, and the table 6 because of their rise in temperature. Due to this radiated heat 11, the front surface 9a of the column which faces the heat source is heated, whereby the temperature of this surface rises also. Furthermore, because of the influence of the environment surrounding the electrical discharge machining apparatus, including the room temperature, the temperature distribution over different parts of the unit may change.
Thus, in the conventional electrical discharge machining apparatus constructed as described above, because the temperature of the components of the machine in thermal communication with the discharge source rises in comparison with other components, for example, as shown by a dot-and-dash line in FIG. 1, the spindle 4 and the front surface 9a of the column undergo thermal deformation, thereby displacing the electrode from its position at the beginning of the machining operation, leading to problems such as a lowering of machining accuracy.
Variation in temperature can also occur in response to changes in room temperature because of differences in heat capacities of the different components of the machining apparatus.
Furthermore, when electric discharge machining is carried out, the discharge energy causes the temperature of the electrode 3 and the machining liquid 2 to rise, the corresponding heat being transmitted to the spindle 4 which holds the electrode 3 in position. Since the spindle 4 is held by the head 8 via a bearing, a guide, etc., (not shown in the figure), heat in the spindle 4 cannot easily be transmitted to the head 8 and the column 9. Moreover, if electrical discharge machining is carried out in an environment where the room temperature can vary greatly, a difference of 2.degree. to 5.degree. C. can occur between the temperature of the column 9, which is exposed to the external atmosphere, and the spindle 4, which is held within the head 8. In this case, as electrical discharge machining of workpiece 5 continues, a difference in temperature is generated between the spindle 4 and the column 9 because of the variation in room temperature and machining heat, whereby a difference in thermal deformation (thermal expansion) occurs between the spindle 4 and the column 9 in the vertical direction, leading to a relative displacement g in the vertical direction between the electrode 3 and the workpiece 5, further decreasing the accuracy in the direction of depth of machining of the workpiece 5.
FIG. 2 illustrates another example of a conventional electrical discharge machining apparatus. Reference numerals in FIG. 2 used commonly in FIG. 1 denote like components. In FIG. 2, reference numeral 14 represents a machining liquid tank which is provided for circulation of machining liquid 2 in the machining tank 1. Reference numeral 13 designates a fan cooler used to regulate the temperature of the liquid. The fan 13, which blows air onto the machining liquid 2 in order to air cool it, is fixed to the machining liquid tank 14.
When electric discharge machining in the unit described above is performed, the temperature of the machining liquid 2 rises due to the discharged energy, and the rise in the liquid temperature heats the machining tank 1 and the table 2 so that the temperatures of the machining liquid 2, the machining tank 1, and the table 6 rise in comparison with the temperature of the ambient air. The fan cooler 13 cools the machining liquid in order to suppress the rise in its temperature. However, when the machining conditions have stabilized, a rise of 4.degree. to 5.degree. C. will have occurred in the machining liquid 2. Because of this, heat 11 is radiated as indicated by arrows in FIG. 2 from the surface of the machining liquid, the machining tank 1, and the table 6, each of which undergoes a rise in temperature. The radiated heat 11 also heats the front surface 9a of the column which faces the heat source. As a result, the temperatures of these surfaces rise, and the temperature distribution in the electrical discharge machining apparatus becomes nonuniform. As discussed above, variations in ambient conditions such as room temperature and the difference in thermal capacities of the respective components of the machining apparatus contribute to the nonuniform temperature distribution. Furthermore, local variations in temperature distribution in the column 9 arising due to the heating of the machining liquid 2 during machining and variations in the temperature distribution in the machining unit as a whole in response to changes in ambient conditions such as room temperature have different origins and occur independently of each other.
Accordingly, during electrical discharge machining operations in the conventional electrical discharge machining unit illustrated in FIG. 2, the temperature of the surfaces of the electrical discharge machining unit that face the heat source rises more than the surface temperatures elsewhere in the unit whereby, for example, as shown by the double-dot/dash line in FIG. 2, thermal deformation occurs on the front surface 9a of the column and the spindle 4, leading, as in the first instance, to relative displacement between the electrode 3 and the workpiece 5 in the vertical direction, causing defects such as a lowering of the machining accuracy of the workpiece 5. Also, when the machining liquid 2 is cooled by the fan cooler 13, the fan cooler 13 itself generates a rise of 4.degree. to 5.degree. C., whereby the precision of machining is still further adversely affected.
FIG. 3 shows a perspective view of a machining apparatus showing a measured distribution of temperature in the apparatus and the amount of the relative displacement between the machining electrode and the workpiece in a room where ventilation inside the machining apparatus is not sufficient. FIG. 4 shows variations in the relative position between the electrode 3 and the workpiece 5 over time as the room temperature changes, the differences in relative position being shown in mutually orthogonal X, Y, and Z directions. FIG. 5 is a chart showing the results of measurements of room temperature and the difference in temperatures at the spindle 4 and the column 9 while normal electrical discharge machining operations are being carried out, while FIG. 6 is a chart showing the results of measurements of the amount of mutual displacement in the vertical direction between the electrode 3 and the workpiece 5 under normal ambient variations over a 24-hour period.
In the conventional electrical discharge machining apparatus, the cooling fan, namely, the air blower 12, can be placed either on top of the column or at the back of the bed in order to blow atmospheric air so as to partially equalize the temperatures of the various components of the apparatus, but this has the defects that the paths of the air flow are not clearly known and only the interior of the machine structure is ventilated.
Furthermore, even if the machining unit is cooled by means of an air blower 12 fixed to the top of the column or at the back of the bed, because of the rise in temperature of the machining liquid 2 during normal operation, temperature differences occur among the workpiece 5, the table 6, and the saddle 7 (which faces the machining liquid 2), and the column 9, the bed 10, etc., cooled by the air blower 12. These differences in temperature afffect the temperature balance in the machining apparatus as a whole, and hence adversely affect the machining accuracy. In other words, during normal machining, components facing the machining liquid 2 eventually reach a temperature nearly as high as the temperature of the liquid itself, whereas the temperature of the other components remains near room temperature, and thus a difference in the amounts of thermal expansion occurs of the various components, which is a major cause of relative displacement between the machining electrode and workpiece. Since the measures applied in the conventional electrical discharge machining apparatus to overcome these difficulties were insufficient, the machining accuracy was insufficient in many cases.